Widespread Use of Optical Access Network
As the Internet is coming into widespread use, there is an increasing demand for high-speed networks. ADSL (Asymmetric Digital Subscriber Line), B-PON (Broadband PON), E-PON (Ethernet PON), and G-PON (Gigabit Capable PON) are in widespread use. Specifically, a PON system includes an optical line terminal (OLT) installed in a station and an optical network unit (ONU) installed in each user's home. An optical splitter splits a single fiber cable from the OLT into users for connection between the OLT and each of ONUs. The PON system can reduce fiber cabling costs and provide high-speed transmission because of optical transmission. The PON system becomes popular for all the countries of the world.
Increase in the Communication Speed and Power Consumption
As frequency bands allocated for users expand, optical access networks are requested to accelerate communication speeds. The IEEE 802.3av standard standardizes 10G-EPON capable of the communication speed at 10 Gbps upstream and downstream (refer to IEEE 802.3av). As communication speeds increase, electronic parts and units such as OLTs and ONUs are expected to consume more electric power.
Requests for Reduction of Power Consumption in Network Devices
Along with the recent trend toward the reduction of environmental burdens, network devices are requested to reduce the power consumption. However, the optical access network tends to increase the power consumption while an increase in optical line subscribers also increases the number of devices and an increase in the communication speed also increases the power consumption per device. Accordingly, optical access network devices require reducing the power consumption. Another possible reason for reducing the power consumption is to ensure the communication infrastructure in the event of a power failure or other disasters. When a power failure occurs, devices are considered to work on batteries. The long battery life is desirable in terms of the ensured communication infrastructure. Therefore, devices need to reduce power consumption.
Conventional Power Saving Technology for PON Systems
A known power saving technology for PON systems is to use the sleep mode for ONUs. The sleep mode changes an ONU to a power-saving state. The ONU operates while switching between a normal mode for normal power consumption and the sleep mode for low power consumption. A known method of sleeping the ONU determines the time to enable and the time to cancel the sleep mode in accordance with communication between the OLT and ONU (e.g., IEEE P802.3av Task Force “3av—0809_mandin—4.pdf”). After the time to cancel the sleep mode, the ONU receives a frame allocated to an upstream band from the OLT and then resumes the normal mode to be ready for upstream data transmission.
Conventional Power Saving Technology for Wireless Systems
As disclosed in JP-T-2003-517741 and JP-A-2004-172772, for example, the power saving methods for wireless systems use the sleep mode for terminal stations. To reduce the amount of delay and the power consumption in the wireless system, a terminal station exits from the sleep mode by transmitting a sleep cancel report to the base station in response to reception of cyclically transmitted notification information.